The present invention relates to the communication of analog and digital channel signals, and more particularly to a frequency allocation technique that minimizes the effects of amplifier distortion in a mixed analog and digital cable television system.
Cable television systems can be built to cover a wide geographic area and/or a large subscriber population by establishing a number of distribution hubs. Signals from the cable headend can be carried to the hubs by coaxial cable, microwave or, more recently, optical fibers. A major advantage in using optical fibers is that the signal transmission quality is greatly enhanced in comparison with coaxial cable distribution, which suffers from distortion introduced by amplifiers that must be provided along a coaxial cable path. Although low distortion "feed forward" amplifiers have been developed for the communication of analog signals via coaxial cable, substantial intermodulation products can result when such amplifiers are used to carry digital signals as well as analog signals. Such intermodulation products result from the mixing of different signals on the cable, and generate interference in the distributed television signals.
With the advent of digital processing techniques, it has become advantageous to distribute certain signals in a digital format. Indeed, it is expected that in the future, digital optical transmission will become the preferred technique for distribution of signals within and between cable systems. At rates of the order of 90 Mbits per second per video signal, digital systems will be able to provide transmission of extremely high and unvarying quality.
As the technology progresses toward total digital communications, it has become advantageous to transmit both analog and digital signals together on a cable television network. For example, certain high quality premium channels may be transmitted in a digital mode, together with other channels in a conventional analog format. However, as noted above, the transmission of digital signals together with analog signals can result in unacceptable harmonic distortions introduced by a coaxial cable distribution path, and particularly by the nonlinearities inherent in present day distribution amplifiers.
It would be advantageous to provide a technique for transmitting a multiplex of digital and analog signals via a cable television distribution network in a manner that minimizes intermodulation distortions between the various television channel signals. It would be further advantageous to provide such a technique that requires a minimal amount of additional hardware, and can be implemented in an efficient and cost effective manner.
The present invention provides a method and apparatus for reducing interchannel harmonic distortions in a multiplex of analog and digital channel signals, enjoying the aforementioned advantages.